Combined cycle power systems and cogeneration facilities utilize gas turbines to generate power. These gas turbines typically generate high temperature exhaust gases that are conveyed into a heat recovery steam generator (HRSG) that produces steam. The steam may be used to drive a steam turbine to generate more power and/or to provide steam for use in other processes.
Operating power systems at maximum efficiency is a high priority for any generation facility. Factors including load conditions, equipment degradation, and ambient conditions may cause the generation unit to operate under less than optimal conditions. Supercharging (causing the inlet pressure to exceed the ambient pressure) turbine systems as a way to increase the capacity of gas-turbine is known. Supercharged turbine systems typically include a variable speed supercharging fan located at the gas turbine inlet that is driven by steam energy derived from converting exhaust waste heat into steam. The supercharging fan is used to increase the air mass flow rate into the gas turbine so that the gas turbine shaft horsepower can be augmented.
A problem with conventional supercharged combined cycle systems is that they are uneconomical due primarily to the prevailing “spark spread.” Spark spread is the gross margin of a gas-fired power plant from selling a given amount of electricity minus the cost of fuel required to produce that given amount of electricity. Operational, maintenance, capital and other financial costs must be covered from the spark spread. Another problem with conventional supercharged systems is that controlling the inlet fan is difficult. In many cases, the return on investment of such systems is not attractive. Conventional supercharged combined cycle systems do not provide customers with sufficient system flexibility, output and efficiency over the system life cycle. Additionally, those systems require significant modifications and are sometimes not compatible with legacy systems.